Thallus: crustose to squamulose, forming a crust-like cover, up to 150 µm thick and 2 cm diam., on a thin, blue-black hypothallus that is often poorly developed in dry habitats squamules: up to 2 mm diam., remaining discrete at the margin of the crust, often (particularly in drier habitats) becoming imbricate and caespitose in central parts upper surface: usually gray-brown (or gray to blackening) upper cortex: paraplectenchymatous with irregularly thickened cell walls, 15-20 µm thick Apothecia: common, up to 1 mm diam., frequently proliferating and forming clusters, with or without a thalline margin (even on the same specimen); disc: brown to blackish (lighter in shade forms), often becoming convex; exciple: variously developed, up to 60 µm thick, subparaplectenchymatous; hymenium: brown-black above, otherwise hyaline, I+ blue-green and turning red-brown, 100-120 µm high asci: clavate to subcylindrical, 8-spored with apical amyloid sheets ascospores: simple, colorless, lacking an epispore, with numerous internal oil droplets that may give the spore the impression of being septate, 13-15 (-17) x 5-6 µm Spot test: all negative Secondary metabolites: none detected. Substrate and ecology: on rocks (often sandstone) by rivers or in forested valleys, most common at middle altitudes (up to 2000 m) World distribution: widespread in temperate parts of the Northern Hemisphere Sonoran distribution: primarily at mid-elevations in Arizona, southern California and Baja California. Notes: It is a variable species, that is sometimes difficult to recognize. The variation in thallus form is mostly environmental. The form with a distinct hypothallus is principally found on shaded, moist rocks in narrow river-ravines. The thicker forms are from rocks (often sandstone) in forests. In its typical form it has discrete, brownish squamules on a thin distinct blackish hypothallus. However, only a few specimens in the Sonoran region correspond to that description. One of them (Nash 38109) surprisingly has a few granular soralia marginally. I interpret this as a local variation rather than representing a new species., although it might be recognized as a form. All other characters are typical of the species. Most specimens form thicker crusts or cushion-like aggregations, morphologically approaching F. californica and F. hookerioides. From the latter species it is easily distinguished in usually having brownish convex apothecia with irregular, often excluded thalline margin. Fuscopannaria californica is definitely thicker, builds conspicuous cushions and has larger spores. The three also occupy different ecological niches.